Sm2Fe17 ALLOY WITH Ta ADDITION AND THE PREPARATION PROCEDURE

Abstract: The process is provided for producing the Sm2Fe17 alloy with Ta addition, without expensive high temperature-long term annealing. As-cast alloy is suitable as the basic material for the preparation of permanent magnets based on nitrides of intermetallic compounds of rare earth and transition metals. The process is described for the as-cast alloy preparation with two phases present, Sm2Fe17 suitable for the nitrogenation to the hard magnetic Sm2Fe17N3-x phase and paramagnetic TaFe2 hexagonal phase. The phase modification is attained with the addition of a third alloying element. Ta substitutes Fe in the Sm2Fe17 nominal composition. The amount of iron or other ferromagnetic phases is lower than 0.5 wt.%. The invention relates to the preparation of the cast alloy with a minimum amount of free iron and/or other soft magnetic phases, which is suitable for further nitrogenation without high temperature long term annealing. The as-cast alloy is suitable for the preparation of the Sm2Fe17-xTaxN3-y compound.

Description: Sm 2 Fe 17 ALLOY WITH Ta ADDITION AND THE PREPARATION PROCEDURE The invention relates to the Sm 2 Fe 17 alloy with Ta addition, and to the process for producing this alloy, which is suitable as the basic material for the preparation of permanent magnets based on nitrides of intermetallic compounds of rare earth and transition metals. Nitrides of binary intermetallic compounds between rare earths, in the following text designated as RE, and transition metals, in the following text designated as TM, thus Sm 2 Fe 17 N x , are new ferromagnetic materials, first discovered by Coey J. M. D. and Sun H., published in J. Mag. Magn. Mat. 87, (1990), L251 (1 ). The preparation of these materials is described in the European applications EP 369097, EP 453270, EP 538643, EP 493019 and Canadian applications CA 2040686, CA 2058283, . Permanent magnets based on the Sm 2 Fe 17 N 3 -x ternary interstitial phase with a rhombohedral Th 2 Zn 17 - type structure are considered to be competitive with the well-known Nd-Fe-B based magnets. The intrinsic properties of Sm 2 Fe 17 N 3 . x , where x is approximately 0.3, include a higher Curie temperature (Tc) for 100 to 200°C and anisotropy field (Ha), and a slightly lover magnetisation saturation (Ms). The compound where RE is Sm possesses these beneficial properties due to Sm, which is the only RE with a preferential vector of magnetisation in the direction of c -axis of the rhombohedral structure, which is the necessary condition for high magnetocrystal anisotropy. For comparison the magnetic properties of Nd 2 Fe 14 B and Sm 2 Fe 17 N x are as follows: Nd 2 Fe 14 B: T c = 588 K, Ms = 1.60 T, Ha = 7.5 T (BH)max = 509 kJ/m 3 Sm 2 Fe 17 N x : T c = 749 K, Ms = 1.55 T, Ha = 14 T, (BH)max = 472 kJ/m 3 Unfortunately, Sm 2 Fe 17 binary phase which is used for production of the Sm 2 Fe 17 N 3 . x interstitial ternary compound through a gas-phase interstitial modification process, forms through a peritectic reaction between primary crystallised iron and Sm- rich liquid: L(liquid) + Fe -> Sm 2 Fe 17 . This always leads to considerable amounts of free iron (up to 25 %.) and some SmFe 2 or SmFe 3 Sm-rich phases in the as cast state. Free iron especially, unless removed by a subsequent isothermal homogenization treatment, reduces the cdercivity of the Sm 2 Fe 17 nitride produced when used for permanent magnets. The usual methods for creating an alloy without soft magnetic phases are either high temperature-long term annealing of the samarium-rich cast alloy (five days at 1050°C) or through the phase modification with the addition of a third alloying element (from the group of transition metals). The phase relation can be influenced by the addition of the third element, which modifies the crystallization and prevents the primary crystallization of free iron and leads to a two phase structure consisting of Sm 2 Fe 1 7 phase and one paramagnetic intermediate Laves phase. Up to the present, research work in this field was focused on the addition of the elements Ti and Nb as elements which influence the phase relations, and is published in the open literature Reinch B., Grieb B., Henig E. T., Petzow G., IEEE Trans. Magn., 28, (1992), 2832 and Platts A. E., Harris I. R., Coey J. M. D., J. Alloys Comp., (1992), 251 Of the other transition elements it was shown that Zr, Mo, Hf, V, Ti and Ta also form intermediate Laves phases of the AB 2 - type. There were no data published or patented on the influence of these elements on the phase relationships in the system Sm-Fe-TM. The technical problem of the preparation of a cast alloy without the formation of unfavourable secondary phases can be partially solved by high temperature-long term annealing of the samarium-rich as -cast alloy, which is an expensive process, or by influencing the phase modification by the addition of Nb or Ti. The purpose and the aim of the invention relates to the Sm 2 Fe 17 alloy with Ta addition and to the process for producing this alloy with the corresponding properties. The production of the alloy should be cheap and comparatively easy. The technical problem of the preparation of the cast alloy without the formation of unfavourable secondary phases is solved in the invention by Ta addition and with the appropriate casting technique. Tantalum forms a Pauli paramagnetic TaFe2 intermediate hexagonal Laves phase with iron, as described in the patent claims. By this invention the preparation of as -cast alloy which contains a minimal quantity of free iron and other ferromagnetic phases is enabled. The as -cast alloy is suitable for the nitrogenation procedure without any subsequent annealing process. As such, the nitrogenated alloy is suitable as a basic material for the preparation of a new generation of permanent magnets of Sm 2 Fe 17 N x type. The following example is meant to illustrate preffered embodiment of this invention. The alloy which contains a minimum quantity of free iron and other ferromagnetic phases, which diminish the hard magnetic coercivity of Sm 2 Fe 17 alloy, is produced. The amount of soft magnetic phases is low enough that for the preparation of Sm 2 Fe 17 alloy suitable for the nitrogenation and production of Sm 2 Fe 17 N x , high temperature annealing can be avoided. By this invention the alloy with the composition Sm 2 Fe 17 . x Ta x is produced by arc melting in an inert atmosphere of Ar purified with Ti as a getter. As -cast alloys were prepared by arc-melting of samples in a Ti purified Ar atmosphere using elemental Sm (99.99 %), Fe (99.99 %) and Ta (99.9%). The Ta addition is between 1 to 5 at.%. Excess Sm was added to the nominal composition of Sm 2 Fe 17 _ x Ta x (10 to 30 at.%) to counterbalance Sm evaporation loses during melting. First Fe and Ta are melted and Sm is added later. The samples are remelted four times to improve macro and micro homogeneity. In some cases three times is sufficient and sometimes the melt has to be remelted several times additionally. Microstructural analysis shows phase compositional changes, which are dependent upon the increasing Ta content of the alloys of nominal composition Sm 2 Fe 17 . x Ta x . From Fig. 1 , which shows the microstructure of the tantalum -free alloy, we can see the normal appearance of the as -cast state of the nominally stoichiometric Sm 2 Fe 17 alloy. Dendrites of primary crystallised iron (black) are surrounded by peritecticaly formed Sm 2 Fe 17 phase (grey) with some Sm -rich phase which was identified as SmFe 2 (white). The microstructural morphology is typical for the as -cast alloy of stoichiometric composition with dendrites of primary recrystallised iron, surrounded by peritecticaly formed Sm 2 Fe 7 phase and partly with the Sm -rich phase, identified as SmFe 2 phase, detrimental for the magnetic properties and which can be removed by the known method of high temperature long term annealing. Figure 2 shows the microstructure of the alloy with a nominal composition of 5 at.% of Ta. No iron dendrites are observed and the microstructure consists of a mixture of only Sm 2 Fe 17 (grey) and TaFe 2 (white) phases. Qualitative confirmation of the presence of the two phases Sm 2 Fe 17 and TaFe 2 in the as -cast alloy were confirmed by thermomagnetic analysis of the samples, which is normally used for Curie temperature determination of the extant phases. The difference between the tantalum -free alloy and 5 at% Ta material is evident from Figure 3, where normalised temperature curves are shown for a) Ta -free alloy, b) alloy with 5 at.% Ta and c) TaFe 2 alloy. For the Ta -free alloy three characteristic steps appeared on the M = f(T) curve, corresponding to the Curie temperatures of Sm 2 Fe 17 , SmFe 2 and Fe respectively. The thermomagnetic curve of the 5 % Ta material shows only one large step, consistent with the Curie temperature of the Sm 2 Fe 17 phase. A thermomagnetic scan of the TaFe 2 phase shows the paramagnetic nature of the phase. Fig.4 shows thermomagnetic curves normalised to the Curie temperature of the Sm 2 Fe 17 phase for the alloys with a) 1 % Ta, b) 2% Ta and c) 4% Ta. Thermomagnetic analysis also shows the increase in the Curie temperature as a function of the Ta concentration in the Sm 2 Fe 17 phase. The invention relates to the preparation of the cast alloy with a minimum amount of free iron and/or other soft magnetic phases, which is suitable for further nitrogenation without high temperature long term annealing. The as -cast alloy is suitable for the preparation of the Sm 2 Fe 17 . x Ta x N 3 . y compound. The most important advantage of the invention is the cheaper preparation procedure of the alloy. Ta addition is a new solution to the problem of the appearance of soft ferromagnetic phases which deteriorate the hard magnetic properties of the alloy.

Claims: Claims. 1. The Sm 2 Fe 17 alloy with Ta addition characterized by the use of 1 at.% to 5 at. % of Ta and Sm and Fe as a basic material. 2. The process for producing the Sm 2 Fe 17 alloy with Ta addition, characterized in that the alloying of Fe and Ta and further addition of 10 to 30 wt. % of Sm and that the alloy is at least remelted three times.

Inventors: EDWARDS PLATTS ADRIAN, HARRIS IVOR REX, KOBE BESENICAR SPOMENKA, SAJE BORIS, EDWARDS PLATTS, ADRIAN, HARRIS, IVOR, REX, KOBE, BESENICAR, SPOMENKA, SAJE, BORIS
Applicants: INST STEFAN JOSEF, INSTITUT "JOZEF STEFAN"
Application year: 1994
Main classification: H01F1
IPC codes: C01F1, H01F1, H01F41